The need of energy is increasing continuously, because of increase in industrialization and population. The basic sources of this energy are petroleum, natural gas, coal, hydro and nuclear. The major disadvantage of using petroleum based fuels is atmospheric pollution created by the use of petroleum diesel. Petroleum diesel combustion is a major source of greenhouse gas (GHG). Apart from these emissions, petroleum diesel is also major source of other air contaminants including NOx, SOx, CO, particulate matter and Volatile Organic Compounds (VOC).
Biomass is one of the better sources of energy. Large-scale introduction of biomass energy could contribute to sustainable development on several fronts, environmentally, socially and economic. Bio-diesel (monoalkyl esters) is one of such alternative fuel, which is obtained by the transesterification of triglyceride oil with monohydric alcohols. Biodiesel fuel can be prepared from waste cooking oil, such as palm, soybean, canola, rice bran, sunflower, coconut, corn oil, fish oil, chicken fat and algae which would partly decrease the dependency on petroleum-based fuel.
Macroalgae has also been used in the production of biodiesel. Microalgae with higher oil content than plants are known and they are faster and easier to grow. Microalgae can provide several different types of renewable biofuels. These include methane produced by anaerobic digestion of the algal biomass, biodiesel derived from microalgal oil and photobiologically produced biohydrogen. The idea of using microalgae as a source of fuel is not new but it is now being taken seriously because of the extinguishing petroleum resources and, more significantly, the emerging concern about global warming that is associated with burning fossil fuels.
Microalgae comprise a vast group of unicellular photosynthetic, heterotrophic organisms which have an extraordinary potential for cultivation as energy crops. Microalgae are the great source of many highly valuable products such as polyunsaturated fatty acids, astaxanthin and bioactive compounds.
Microalgae can be grown in two different modes: Photoautotrophic and Heterotrophic mode of growth. Large-scale production of these products, however, has hindered by an ability to obtain high cell densities and productivities in photoautotrophic systems because of light penetration issues and uncontrolled growth conditions. High cell density processes suitable for heterotrophic cultures of microalgae may provide an alternative means for large-scale production of algal products of high value. The heterotrophic growth of algae holds many practical applications in industrial scale especially in a controlled manner to obtain highest biomass as well as lipid productivity.
In heterotrophic conditions algae can be grown on organic carbon sources, such as sugars and organic acids. This mode of culture eliminates the requirement for light and therefore, offers the possibility of greatly increased cell density and productivity. Some microalgae show rapid heterotrophic growth. Heterotrophic algal cultivation has been reported to provide not only a high algal biomass productivity, but high cellular oil content as well. Additionally the culture suffers from the contamination by undesired microbes. However, to date, the very few reports of such processes for microalgal cultivation have mostly been on lab-scale work/plant scale.
Xu et al 2006 (Han Xu, Xiaoling Miao and Qingyu Wu High quality biodiesel production from a microalga Chlorella protothecoides by heterotrophic growth in fermenters. Journal of Biotechnology Volume 126, Issue 4, 1 Dec. 2006, Pages 499-507) discussed high quality biodiesel production from a microalga Chlorella protothecoids through the technology of transesterification. The technique of metabolic controlling through heterotrophic growth of C. protothecoides was applied, and the heterotrophic C. protothecoides contained the crude lipid content of 55.2%. To increase the biomass and reduce the cost of alga, corn powder hydrolysate instead of glucose was used as organic carbon source in heterotrophic culture medium in fermenters. The result showed that cell density significantly increased under the heterotrophic condition, and the highest cell concentration reached 15.5 g L−1. Large amount of microalgal oil was efficiently extracted from the heterotrophic cells by using n-hexane, and then transmuted into biodiesel by acidic transesterification. The biodiesel was characterized by a high heating value of 41 MJ kg−1, a density of 0.864 kg L−1, and a viscosity of 5.2×10−4 Pa s (at 40° C.). The method has great potential in the industrial production of liquid fuel from microalga.
Li et al. 2007 reported (Li X F, Xu H, Wu Q Y (2007) Large-scale biodiesel production from microalga Chlorella protothecoides through heterotrophic cultivation in bioreactors. Biotechnol Bioeng 98:764-771) an integrated approach of biodiesel production from heterotrophic Chlorella protothecoides focused in bioreactors. Through substrate feeding and fermentation process controls, the cell density of C. protothecoides achieved 15.5 g L−1 in 5 L, 12.8 g L−1 in 750 L, and 14.2 g L−1 in 11,000 L bioreactors, respectively. Resulted from heterotrophic metabolism, the lipid content reached 46.1%, 48.7%, and 44.3% of cell dry weight in samples from 5 L, 750 L, and 11,000 L bioreactors, respectively.
Liang et al (2009) reported (Liang Y, Sarkany N, Cui Y 2009 Biomass and lipid productivities of Chlorella vulgaris under autotrophic, heterotrophic and mixotrophic growth conditions. Biotechnology Letters July; 31(7):1043-9) biomass and lipid productivities of Chlorella vulgaris under different growth conditions. While autotrophic growth did provide higher cellular lipid content (38%), the lipid productivity was much lower compared with those from heterotrophic growth with acetate, glucose, or glycerol. Optimal cell growth (2 g l(−1)) and lipid productivity (54 mg l(−1) day(−1)) were attained using glucose at 1% (w/v) whereas higher concentrations were inhibitory. Growth of C. vulgaris on glycerol had a similar dose effects as those from glucose. Overall, C. vulgaris is mixotrophic.
US patent application 2009/0211150A1 discloses a method to produce biodiesel from algae using a strain of microalga chlorella protothecoids, by screening a specific strain with characteristics of high yield of biomass and high oil content, cultivating the screened strain for high-cell-density growth for up to 108 grams of dry cell weight per liter of the suspension in a bioreactor using solutions containing carbohydrates as feed, harvesting and drying the high density cultivated algal cells to extract oil from the dried algal cells, and producing the biodiesel by catalyzed transesterification using the extracted oil as feedstock.
In the prior art few microalgal strains have been cultured to produce lipids for biodiesel. However, lipids produced from these microalgal strains are mostly rich in unsaturated fatty acids which makes them unsuitable for biodiesel production. Therefore, there exists a need to have increased saturated fatty acids content in lipids from microalgal strain with higher biomass oil content using various cheap carbon sources for cultivation which ultimately makes the process more cost effective and applicable in terms of its industrial success.